The present invention relates to the magnetic resonance art. It finds particular application in conjunction with magnetic resonance imaging of the head and neck regions of human patients and will be described with particular reference thereto. It is to be appreciated, however, that the invention is also applicable to the examination of other portions of the human anatomy and to the imaging or spectroscopic examination of non-human subjects.
In magnetic resonance imaging, a subject is typically positioned within a strong, temporally constant magnetic field. A series of spatial location encoding magnetic field gradient pulses are applied across a region of interest within the magnetic field. Radio frequency pulses are applied for inducing and manipulating magnetic resonance of dipoles in the region of interest. A radio frequency receiving coil is positioned to receive radio frequency magnetic resonance signals emanating from the region of interest.
Radio frequency receiving coils of various types have been utilized. The radio frequency coils may be disposed to receive signals from the imaging area as a whole or may be positioned closely adjacent the surface of a body portion to be imaged, such as the head. These receiving coils include the "birdcage" type receiving coil. See, for example, Hayes U.S. Pat. No. 4,692,705 issued Sep. 8, 1987. Birdcage coils resonate with cosinusoidal current distribution. Variations on the birdcage coil include end-capped birdcage coils and elliptically end-capped birdcage coils. Other types of receiving coils include coils with cosine distributions, spherical resonators, dome coils, and the like.
The birdcage coil designs have a generally Gaussian distribution of B.sub.1 magnetization along the coil axis, with the maximum field strength at the coil center. For head imaging, such a distribution is not ideal. Although the Gaussian distribution can be flattened in the central region, the most commonly imaged region of the human head, the circle of Willis and the surrounding area, are offset from the center of a head coil. Although longer birdcage coils may have a longer area of uniformity, they also have a lower sensitivity. Further, a longer birdcage coil does not allow centering of a short adult's brain at the coil center. A shorter birdcage coil suffers from poorer homogeneity.
End-capped birdcage coils tend to improve the signal-to-noise ratio and homogeneity near the end cap region towards the top of the patient's head. The end-capped birdcage provides a slightly improved signal-to-noise ratio and slightly improved homogeneity towards the closed end. However, the current distribution in the end cap is not alterable. Therefore, the B.sub.1 field distribution along coiled axes and close to the end cap remain unchanged. Also, the B.sub.1 field computation in the end cap vicinity is complicated by the presence of the continuous cap or shield. The elliptical birdcage is best suited for use with local gradients and for covering the region between the top of the head to the middle of the cerebrum. Multiple birdcage coils mounted end-to-end tend to provide a uniform magnetic field at the coil center with sharp cut-offs toward the coil ends. Although connecting a plurality of birdcage coils provides a uniform field at the coil center with a sharp cut-off adjacent both ends, this is at the expense of the B.sub.1 field strength at the coil center. In the multiple birdcage coils, the two outer structures carry a maximum current with only a fraction of their current flowing in the inner structures at the resonant frequency. This causes a reduction in the B.sub.1 field at the coil center. This compromises the signal-to-noise ratio for attaining a high degree of B.sub.1 uniformity at the coil center.
A two-dimensional ladder type network can improve sensitivity and homogeneity over portions of the brain. See, for example, Derbey U.S. Pat. No. 5,315,251, issued May 24, 1994.
Although two-dimensional networks provide a slightly improved sensitivity and homogeneity relative to a standard birdcage coil over top portions of the brain, the field along the coil axis reduces rapidly when nearing the closed end of the coil. Higher power is needed in the radio frequency excitation or flip pulses along the coil axis. A rapidly reducing radio frequency field or RF gradient is created which is undesirable for imaging smaller vessels where the focus is more towards the top of the brain such as in or above the circle of Willis and in superficial areas of the brain in the case of functional imaging.
The present invention provides a new and improved radio frequency coil which overcomes the above-referenced problems and others.